Paint solid color and local attachment backgrounds into scrolling layer.

third_party/WebKit/Source/core/paint/PaintLayer.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 2448 matching lines @@
 
 bool PaintLayer::shouldBeSelfPaintingLayer() const
 {
     if (layoutObject()->isLayoutPart() && toLayoutPart(layoutObject())->requiresAcceleratedCompositing())
         return true;
     return layoutObject()->layerTypeRequired() == NormalPaintLayer
         || (m_scrollableArea && m_scrollableArea->hasOverlayScrollbars())
         || needsCompositedScrolling();
 }
 
+bool PaintLayer::shouldPaintBackgroundOntoForeground() const
+{
+    return scrollsOverflow()
+        && layoutObject()->style()->hasEntirelyLocalBackground()
+        && !stackingNode()->hasNegativeZOrderList();
+}
+
 void PaintLayer::updateSelfPaintingLayer()
 {
     bool isSelfPaintingLayer = shouldBeSelfPaintingLayer();
     if (this->isSelfPaintingLayer() == isSelfPaintingLayer)
         return;
 
     m_isSelfPaintingLayer = isSelfPaintingLayer;
 
     if (PaintLayer* parent = this->parent()) {
         parent->dirtyAncestorChainHasSelfPaintingLayerDescendantStatus();
@@ skipping 459 matching lines @@
 
 void showLayerTree(const blink::LayoutObject* layoutObject)
 {
     if (!layoutObject) {
         fprintf(stderr, "Cannot showLayerTree. Root is (nil)\n");
         return;
     }
     showLayerTree(layoutObject->enclosingLayer());
 }
 #endif